Description

While in your inventory, with items in your hotbar, quickly hitting the numbers while in creative mode can cause some items to overwrite others, and create a duplicate of one item, while removing the other. This does not work in survival.

Spam-clicking items causes same effect. Can sometimes disappear even in survival, see MC-4108.

Easy way to reproduce:

Open the inventory

Put two different items in slots 1 and 2

Hold the mouse over slot 2

Press and hold "1"

The items will swap a while and eventually the item of slot 1 overwrites the item of slot 2